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Absolutely I'd expect his BABIP to drop back -- though not all the way to career norms, if he is in fact hitting his prime right now. On the other hand, I would expect Safeco to favor him more than Nationals Park does.
Drop back to where? Dunn already had 141-146 OPS+ seasons in Cincy, and his batting eye is better now than it was then.
His BABIP is worth considering, but in the big picture, you're talking about a hulking 250-lb. lefty who hits 40+ home runs every season, whose eye is getting better and whose performances are improving.
His 2010-11 seasons won't match Junior's best ever, but I expect them to fit into Junior's top 10 seasons.